1. Field of the Invention
The invention relates to an ultra-high pressure mercury lamp of the short arc type in which the mercury vapor pressure during operation is 15 MPa to 20 MPa. The invention relates especially to an ultra-high pressure mercury lamp which is used as the light source of a liquid crystal display device and a DLP(copyright) projector device (Texas Instruments: digital light processing) using a DMD (digital mirror device).
2. Description of the Prior Art
A high pressure discharge lamp, such as an ultra-high pressure mercury lamp or the like, has been used recently as the light source of a projector device of a liquid crystal projector device or the like, in which at least 0.15 mg/mm3 of mercury is added as the emission substance and in which the operating pressure during operation is, for example, 15 MPa to 20 MPa, and therefore, is higher than in other types of discharge lamps.
In such a high pressure discharge lamp, the light emitting part which forms a discharge space in which there are opposed tungsten discharge electrodes, is filled with halogen together with emission substances, such as an inert gas, mercury, and the like, in order to prevent the phenomenon of devitrification as a result of blackening of the envelope and as a result of milky opacification of the envelope of this light emitting part or a similar phenomenon from occurring.
The process of hermetic sealing in the above described high pressure discharge lamp is advantageously a shrink seal method in which the inside of, for example, a silica glass tube, which has a light emitting part and sealing parts, is exposed to a negative pressure. The outer periphery of the respective sealing part of this silica glass tube, in this state, is heated by means of a torch or the like. The diameter of the silica glass comprising the envelope of this sealing part is reduced by softening. In this way, hermetically sealed parts are formed.
The above described high pressure discharge lamp generally does not have any residue of an outlet tube in order to ensure intensity of pressure tightness in lamp operation. Mo (molybdenum) material is used for metal foils and outer lead pins of the sealing parts.
The halogen introduced into the light emitting part is added, for example, according to the disclosure in Japanese patent disclosure document HEI 2-148561 (U.S. Pat. No. 5,109,181) in the form of CH2Br2 and according to the disclosure in Japanese patent disclosure document HEI 11-297268 in the form of a metal halide. If the halogen is added in this form, at least one of the sealing parts is hermetically sealed in an atmosphere in which the halogen is present, since there should not be any residue of the outlet tube. The Mo present in the sealing parts reacts at the temperature used in the operation of hermetic sealing (roughly 1600xc2x0 C.) with halogen (for example, Br (bromine)) or O2 (oxygen) which remain in the sealing parts, and which remain as a compound, such as MoBr4, MoO3 or the like in the light emitting part.
Gradually, it was found by the research of the inventor and his employees that the Mo compounds which reach the light emitting part in this way adhere to the inside surface of the light emitting part and become the starting point of formation of lamp devitrification during several hours of operation because at the temperature of the light emitting part during lamp operation, it has a lower vapor pressure than W (tungsten) compounds. This means that, as a result of the fact that Mo compounds as the starting point produce devitrification and allow it to grow, the light flux of the lamp is greatly attenuated.
The primary object of the present invention is to devise an ultra-high pressure mercury lamp in which, even in the case of lamp operation over a long time, devitrification of the light emitting part is suppressed.
This object is achieved, according to a first aspect of the invention, in an ultra-high pressure mercury lamp in which, in a silica glass light emitting part, there is a pair of opposed electrodes and the light emitting part is filled with a rare gas, halogen and at least 0.15 mg/mm3 of mercury, in that the amount of halogen in the light emitting part is at least 1.0xc3x9710xe2x88x924 xcexcmol/mm3 and that, moreover, the amount of Mo in the light emitting part is at most 0.5xc3x9710xe2x88x925 xcexcmol/mm3.
The object is achieved according to another aspect of the invention in an ultra-high pressure mercury lamp in which, in a silica glass light emitting part, there is a pair of opposed electrodes and the light emitting part is filled with a rare gas, halogen and at least 0.15 mg/mm3 of mercury, in that the amount of halogen in the light emitting part is at least 2.0xc3x9710xe2x88x924 xcexcmol/mm3 and that, moreover, the amount of Mo in the light emitting part is at most 1.0xc3x9710xe2x88x925 xcexcmol/mm3.
The invention is further described below using one embodiment shown in the drawings.